This invention relates to multiwall bags of the type commonly referred to as xe2x80x9cblock bottom bags,xe2x80x9d and more specifically, to such a bag designed so that the entire interior surface area of the bag that is exposed to product contained in the bag is covered with a plastic liner.
Block bottom bags, which are also commonly called xe2x80x9csatchel bottomxe2x80x9d bags are very useful for holding bulk quantities of material and as such are used ubiquitously in numerous industries. While there are numerous names that are used to describe the bag, they are characterized as having multiple paper plies and a plastic liner, typically polyethylene, and a folded block bottom that is flat when the bag is filled. The open top end of such bags may be closed in several ways, for example by folding it over and xe2x80x9cpinchingxe2x80x9d it shut to seal the bag. When the open end of such bags is closed with a pinched seal the bag is called a xe2x80x9cpinch block bottom bag.xe2x80x9d
Block bottom bags are manufactured from roll stock paperxe2x80x94typically a standard kraft paper, and roll stock polyethylene sheet material. Although there are several well-known processes according to which the bags may be made, briefly described, the bags are fabricated by laminating or gluing a polyethylene layer to an innermost layer of paper. Several additional layers of paper, some of which have been cut and scored by knife blades, are then bonded (as by gluing) to the inner paper layer. Each layer of paper and the plastic layer are xe2x80x9csteppedxe2x80x9d or laterally offset relative to the next adjacent layer as they are laid down to produce a continuous sheet of a flattened material that typically has three or more layers of paper, and a layer of plastic. Rotating knife blades cut perforations in the blank prior to forming the blank into a tube. The blades are registered relative to one another to produce a stepped pattern for each of the layers in the longitudinal direction. The tube is then bottomed, which is the step in the process where the block bottom is formed and the bottom of the individual bags are closed and sealed. The bottoming process involves tucking or folding the sides of the bag inwardly and closing and gluing the bottom flaps over the folded-in sides. The opposite end of the individual bags are left open, to be closed and sealed later by, for example, the customer after filling. For ease of reference herein, block bottom bags are referred to as BB bags.
After the block bottom is formed, the open end of BB bags may be sealed according to several known processes, for example by folding over the top layers and heat-sealing the plastic liner to the outer ply of paper. Other process steps may often be used as well, for example, printing the outer surface of the outer layer of paper with various information such as the identity of the product that will be held in the bag, the manufacturer of the product, and the net weight of the bag.
BB bags offer numerous advantages. These include that the empty bags are flat and many empty bags may be stacked into a relatively small space. This reduces the costs of shipping many empty bags from the manufacturer to the user. The bags are well suited to holding bulk dry materials such as granulated products. The bags are easily filled, and once filled are easily sealed. Once sealed, the bags are strong and typically avoid sifting or leakage, although many conventional BB bags advantageously allow for some migration of air into and out of the sealed bag. Moreover, filled bags have a squared bottom and flat non-gusseted sides that makes the bags particularly suitable for stacking when filled. As such, the filled bags are easily layered in stable stacks on pallets. And in addition to the advantages just described, BB bags are quite economical to manufacture and use compared to other known bulk bags.
But despite the many advantages of BB bags, there are also several problems. One complaint is that conventional BB bags have sharp corners in the bottom area that may be prone to puncture and other damage caused by impact. One solution to this problem is offered in U.S. Pat. No. 5,553,943, which discloses a BB bag that includes forming a sealed bottom end of the plastic liner that is freely detachable from the paper plies. This construction provides a bag that is air-tight when filled and sealed, and is said to avoid problems caused by the sharp corners found in standard BB bags. However, while the bag disclosed in the ""943 patent is useful in many instances, it is not appropriate for use in all situations where bulk materials need to be packaged. For example, in some situations an airtight bag is not desired. Also, the inner plastic liner in a bag such as that described in the ""943 patent may not conform to the shape of the block bottom when the bag is formed and filled. This can result in the bag not standing upright during processing between the filling station and the next operation. Since most filling operations are highly automated, such bag handling failure can be a significant problem. Moreover, the bag of the ""943 patent may be more expensive to manufacture and may require specialized equipment compared to conventional BB bags.
Another, more significant problem inherent in conventional BB bags has to do with paper that remains exposed to the interior of the bags, and thus to the product contained in the bag. Because as noted BB bags are manufactured with the paper laid down in stepped fashion, when the bottom of conventional bags is formed, there are necessarily two relatively small tabs of paper that remain exposed on the interior of the bag. Stated in another way, owing to manufacturing constraints in making a conventional stepped bag, there are two small tabs of paper on the interior bottom of the bag that are not covered with the polyethylene liner, thus exposing paper to the interior of the bag.
This structural characteristic of conventional BB bags is known to cause several problems. First, when the bags are used for agricultural commodities, exposure of the commodity to paper is a concern with regard to contamination. The polyethylene sheet material that is used to line BB bags is less prone to contamination, such as bacterial contamination than paper. It is undesirable to have an agricultural product exposed to paper that typically is not sterilized.
Further, paper can absorb oils and other fluid components that might be present in the dry bulk products, while polyethylene typically will not absorb oils and other fluids. Absorption of oils and the like from the product into the paper can result in damage to the bag and damage to the product.
Accordingly, in view of the shortcomings inherent in conventional BB bags, there is an opportunity to supply a bag that has all of the advantages of conventional bags, yet avoids the shortcomings.
The illustrated embodiment of the present invention is just such a bag. Most significantly, the bag of is constructed in a manner that it eliminates the paper tabs exposed to the interior of the bag that are notorious in conventional BB bags. Product contained in a bag constructed according to the present invention is exposed to only the polyethylene liner and does not touch the paper plies. This provides substantial advantages from a product quality perspective and may improve sanitation of the product contained in the bag. The bag is equally as economical to manufacture as prior conventional BB bags, and may be manufactured with standard equipment used to make conventional bags, with appropriate modifications.
The bag of the present invention is constructed with multiple plies of paper such as kraft paper that are glued together and stepped relative to one another. The layer of kraft that is adjacent what will become the interior of the bag has a layer of plastic laminated onto it.
The bag is formed from a planar, multi-layer sheet of blank material. Each layer in the blank is stepped in the lateral or cross-bag direction. The layers are also stepped in some portions of the bag in the longitudinal direction. The knives that cut the continuous multi-layer material into individual planar blanks from which the individual bags are formed have been modified so that the knife registry creates specific offset or step patterns in the longitudinal direction. As a result of the offset or stepped pattern in the blanks, when the block bottom is formed in the bag, only plastic liner is exposed to the interior. A special step formed in the bag prevents leakage or xe2x80x9csifting.xe2x80x9d